The present invention generally relates to a continuously variable transmission, and more specifically to a method and apparatus for operating a continuously variable transmission in a output torque limited region near zero output speed.
Many work machines, particularly earth working machines, use a continuously variable transmission to drive traction wheels or tracks which propel the work machine. Typically, a hydrostatic transmission, consisting of a variable speed hydraulic pump and a hydraulic motor, provides a continuously variable speed output to the wheels or tracks of the work machine. In particular, the speed output can be continuously varied by controlling the displacement of the hydraulic pump.
An engine provides power to the transmission which controls speed and torque at the power level. For example, the transmission can increase output torque output by decreasing the output speed. On the other hand, the transmission can decrease output torque output by increasing the output speed.
Continuously variable transmissions are capable of providing an output at any speed within its operating range. Therefore, the continuously variable transmission can provide extremely large amount of output torque near zero output speed. In fact, this output torque may exceed the capability of various components of the transmission and drivetrain. To prevent the failure of these transmission and drivetrain components, output torque must be limited near zero output speed.
In the hydrostatic transmission, output torque is limited near zero output speed by a pressure cutoff device. This pressure cutoff device reduces the displacement of the variable displacement pump when the pressure within the hydrostatic transmission exceeds a threshold value. Thus, the transmission ratio is automatically reduced whenever the pressure within the hydrostatic transmission exceeds the threshold value.
A drawback to using the pressure cutoff device is that it may reduce the effectiveness of the transmission controls when the displacement of the variable displacement pump is limited. For example, when the transmission is experiencing a large load, the controls may implement an underspeed algorithm which limits the ratio of the transmission to prevent the engine from stalling. However, when operating within a output torque limited region caused by the operation of the pressure cutoff device, the change in displacement caused by the pressure cutoff device will cause the engine to operate as if the load is reduced. In fact, if no adjustment is made to the underspeed control, the transmission may not be able to react quickly to new loads that would cause the engine 12 to lug.
What is needed therefore is a method and apparatus for operating a continuously variable transmission within the torque limited region near zero output speed which overcomes the above-mentioned drawbacks.
In accordance with a first aspect of the present invention, there is provided an apparatus for controlling a continuously variable transmission. The apparatus includes an input shaft driven by an engine and an output shaft. The apparatus further includes an operator input for generating speed commands and a controller operable to receive the speed commands and generate transmission ratio commands which control a ratio of a speed of the output shaft to a speed of the input shaft. Torque supplied to the output shaft is limited in a torque limited region near zero speed of the output shaft. The transmission ratio command is modified in the torque limited region.
In accordance with a second aspect of the present invention, there is provided a method for controlling a continuously variable transmission. The transmission includes (i) an input shaft driven by an engine, (ii) an output shaft, (iii) an operator input for generating speed commands, and (iv) a controller operable to receive the speed commands and generate transmission ratio commands which control a ratio of a speed of the output shaft to a speed of the input shaft. The method includes the steps of limiting the torque supplied to the output shaft in a torque limited region near zero speed of the output shaft and modifying the transmission ratio command in the torque limited region.